Disorder at the border

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Science  02 Nov 2018:
Vol. 362, Issue 6414, pp. 525-526
DOI: 10.1126/science.aav2019

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How does an ensemble of closely packed atoms rearrange when a solid undergoes a morphological change? Do the atoms follow a common, synchronized path, or do they move independently into their new positions? These elusively simple questions underpin many key problems in modern condensed-matter physics and also affect fields as far removed as soft-matter research and biological sciences. On page 572 of this issue, Wall et al. (1) use the Linac Coherent Light Source x-ray free-electron laser (2) to provide decisive new information on the nature of a structural phase transition. They study the time evolution of the monoclinic phase of crystalline vanadium dioxide (VO2) after it is destabilized with light (35). Previous studies have assumed that this photo induced structural transition proceeds coherently, that is, as a concerted structural rearrangement in which all atoms move at once (6, 7). Wall et al. turn this notion on its head and find a prompt increase in disorder immediately after photoexcitation, long before the material heats.